The purpose of this new project is to identify the serum factors responsible for the stimulation and termination of hepatocyte proliferation following partial hepatectomy of rats. At present the research is focused on the purification and biological and structural characterization of a putative polypeptide isolated from the serum of partially hepatectomized Fischer 344 male rats. A factor of molecular weight 70-120 kD which produces a dose-dependent stimulation of DNA synthesis in cultures of primary hepatocytes has been partially purified using gel filtration and heparin-affinity chromatography. The stimulatory factor is stable for over a month at -70 degrees C. Some activity is lost by treatment with acid at pH 4 followed by dialysis (10 kD mw-cutoff) suggesting the factor may be bound to a large mw macromolecule. The partially purified serum component is also active in cultures of normal rat kidney cells, but is not active in cultures on nonmalignant liver cells isolated from a 12-day old male Fischer rat or in cultures of hepatoma cell lines. Further purification and characterization of this serum component is currently being undertaken. In addition, the investigation of serum obtained from hepatectomized rats for the presence of further stimulatory components or absence of inhibitory factors will be continued. These studies will be extended by investigating conditioned medium from hepatoma cell lines for the presence of similar growth-stimulating polypeptides in order to examine the possible involvement of the serum factors in the neoplastic process.